Method of growing semi-conductors



July 8, 1958 W.'I..ANDAUER ET AL 2,842,467

METHOD OF GROWING SEMI-CONDUCTORS Filed April 28, 1954 FIG. 1 0 .2

FIG. 3

l FIG 5 6 N i P 5E I P I N I l I 0?. F|G.4 I I I u- O I I O o I ITIME ZX Y '2 9w 'g'i' A1 INVENTORS m5 ROLF-W. LANDAUER 5 BY LLOYD PHUN ER w i}SPECIFIC RESISTANCE ATTORNEY Rolf W. Landaue" N. 5. assi oyal 1P.Hunter, Poughheepsie, Easiness lwl'nchines l. N. L, corporation of New1., set-er No. -'.Z6,27ll

er. i is-rs This invention relates to the fabrication of semi-conductorcrystals, and more particularly to the control of the distribution ofselected impurities added to semiconductor crystals.

The semi-conductors used in rectifiers and transistors are made from amaterial of an extremely high degree of purity to which have been addedvery closely controlled quantities of selected impurities, and which hasbeen grown into a single crystal. The concentration of these impuritiesin the crystal determines the desired electrical characteristics of thesemi-conductor material. There are two basic types of these impurities,each imparting to the material certain unique electricalcharacteristics. The first type of impurity, comprising, preferably,elements of the fifth column of the Periodic Table which are Phosphorus,Arsenic, Antimony and Bismuth, is known as the donor type, and producessemiconductor material known as N material. The second type of impurity,comprising, preferably, elements of the third column of the PeriodicTable which are Boron, Aluminum, Gallium and Indium, is known as theacceptor type and produces semi-conductor material known as P material.An excess of one type of impurity over the other determines whether thematerial will exhibit P or N type unique characteristics, and thedifference between the number of donors and aceptors present in a givenquantity of semi-conductor material determines a general electricalcharacteristic, namely, specific resistance. Single semi-conductorcrystals having alternate regions of P and N type material possessunique characteristics that are useful in transistor applications. Theinterface between the regions of P and N type material is known as a PNjunction, Another unique characteristic of semi-conductor material isknown as the Excess Carrier Lifetime which is a measure of the speedwith which the material will return to equilibrium when subjected tothermal .or electrical excitation. The theories that have been advancedto explain these characteristics are well known and need notbe'discussed here. In the, applications of semi-conductors, singlecrystals having various combinations of these characteristics aresometimes desired.

One method of growing a single crystal of semi-conductor material in useat the present time is performed as follows: A seed crystal of thesemi-conductor material is immersed to a depth slightly below thesurface of a quantity of molten semi-conductor material. This moltenmaterial is kept just a few degrees above its melting point while theseed crystal is withdrawn slowly, permitting the material of the melt tosolidify on the face of the seed crystal. Semi-conductor material of theP or N type is formed by adding to the melt of pure material, amounts ofP or N type impurity, as desired. Specific resistance is controlled bythe quantity of impurity added in relation to the quantity of melt, and

excess carrier lifetime is also controlled in this manner. The P-Njunctions are formed by changing the predominance of one type ofimpurity over'the other in the melt after a section of crystal of theinitial type has been grown. This may be done in two ways, the first isto 'use a seed crystal of one type of material and grow a crystal from amelt of the other type of material, and the second is to add a quantityof one type of impurity, grow a length of crystal, and then add agreater quantity of the other type of impurity. The technology involvedin the fabrication of this material is very complex and requires greatskill. For example, impurity concentrations on the order of one impurityatom per ten millions of regular atoms can produce significant changesin the characteristics of the material, thus, the quantities of theseimpurities cannot be regulated by the usual chemical means. The degreeof purity required of the semi-conductor material to be used as acrystal is greater than can be achieved by spectroscopic means. At themelting temperature, impurities can be absorbed from the'atmosphere,hence these crystal growing operations are usually performed undercontrolled atmospheric conditions, or in a vacuum.

It is to the simplified and regulated control of the concentration ofimpurities in selected regions of the semiconductor crystal that thisinvention is directed.

The primary object of this invention is to providea close and simplifiedcontrol over the distribution of impurities in semi-conductor crystals.

Another object of this invention is to provide a simplified method offorming P-N junctions in semi-conductor crystals.

Another related object is to provide a reliable and simplified method ofcontrolling the specific resistance of semi-conductor crystals.

Still another related object is to provide a method of obtaining asemi-conductor crystal having a given excess carrier lifetime.

Other features of the invention will be pointed out in the followingdescription and claims and illustrated in the accompanying drawings,which disclose, by way of example, the principle of the invention andthe best mode which has been contemplated of applying that principle.

In the drawings:

Figure l is a schematic diagram of a system for performing theinvention.

Figure 2 is an illustration of a semi-conductor crystal grown by themethod used in this invention.

Figure 3 is an illustration of a segment of the crystal in Figure 2 cutlengthwise.

Figure 4 is a graphic representation of the variation of specificresistance with impurity concentration in a semi-conductor crystal andFigure 5 is a graphic representation of the variation of impurityconcentration with time in the presence of a varying electric field.

Referring now to the drawings and more particularly to Figure l, acircuit is shown that may be used to carry out the invention; Thecircuit is connected to equipment used in one method of crystal growingwhich, in this instance, is used for illustration only, and theapplication of the invention should not be limited to this type ofcrystal growing operation because the specific invention the movement ofimpurities by an electric field may be applied in other ways to obtain asemi-conductor crystal.

The crystal growing equipment comprises a crucible I mounted upon aninsulating block 2 carried by a supporting frame 3. Contained within thecrucible is a semi-conducting material 4 comprising a metallic elementsuch as germanium or selenium with closely controlled quantities ofimpurities for example those from either or induction coil 6 surroundingthe crucible and connected to a suitable source of alternating current7. A seed crystal dis supported by a holder 9 so that the lower end ofthe crystal engages the upper surface of the melted material 4, and thecable It is connected to the holder 9 and extends over pulleys ll to amember 12 which is slidably mounted on the frame 3. Fixed to a flange 14on the frame 3 is a motor 15 which drives a shaft 16 7 having threadedengagement with the member 12.

While the crystal 8 engages the surface of the material 4, a D. C.voltage is applied across them in one direction or the other, Toaccomplish this, a conductor 18 is connected from one terminal of areversing switch 19 to the holder 9 which is made of conducting materialand a conductor 20 is connected from another terminal of the switch to acontact member 21 engaging the material 4 at the bottom of the crucible.A reversible switch contact member 22 is connected by a conductor 24 toone side of a D. C. source 25 and is connected by a conductor 26 and avariable resistance 27 to the other side of the D. C source.

In a crystal growing operation, the semi-conductor material 4 is kept ina melted condition by the heating coil 6. The seed crystal 8 is immersedin the melt 4 and then slowly lifted.

This is done by the motor 15 turning the threaded shaft 16 to draw theslidable member 12 downwardly and pull the cable 14) over pulleys ill toraise the crystal holder 9 and the seed crystal 8. As the seed crystal 8moves upwardly, the temperature of the melt at the lower end of thecrystal drops and the semi-conductor material freezes on the end of thecrystal. The speed of moving the seed crystal upwardly is a matter oftechnique, varied by many influencing conditions such as temperature andatmosphere, generally this speed is in the vicinity of .050 inch persecond. The appearance of a crystal just after it has been grown is likethat shown in Figure 2.

As the crystal is grown, the switch member 22 is closed in one directionor the other to apply an electric field across theseed crystal 8 and themelt 4. This field has the effect of moving the impurities and changingthe distribution of impurities in the growing portion. ;It has beenfound that the distribution varies with the type of impurities in themelt and also with the intensity of the electric field. When the switchcontact member 22 is moved to one of its closed positions and left thereduring the complete growing of a crystal, a segment (Figure 3) of thiscrystal, taken along the line A A in Figure 2 will exhibit a steadydecrease in specific resistance with a corresponding increase inconcentration of impurities as indicated in Figure 4. By determining thespecific resistances of crystals grown with different types and amountsof impurities present, and with different voltages applied across themelt and the seed crystal during growing operations, it is possible tocalibrate the system so that a crystal may be grown with any desiredspecific resistance. A crystal grown while impurities from the fifthcolumn of the periodic table predominate will be of the type, and thatgrown while impurities from the third column of the periodic tablepredominate will be P type. If impurities from both columns are presentin the mix during the growing operation, the impurities from one columnwill predominate when the electric field is applied across the melt andthe seed crystal in one direction, and those of the other column willpredominate when the electric field is altered either in direction orintensity or both.

P-N junctions in semi-conductor crystals are produced by causing theconcentration of one type of impurity, either P or N type to predominatewhile one section of a crystal is being grown and then causing theconcentration of the other type of impurity to predominate over that ofthe first during the growing of another region of the crystal. Thisalternating predominance of one type of impurity over the other may beaccomplished by using the electric field to move the impurities andthereby produce an excess of the one type over the other in alternateregions of the crystal. The two types of impurities react to an electricfield of a given polarity either by being influenced in the samedirection at different speeds or by each type of impurity beinginfluenced in opposite directions. The manner in which this reactiontakes place has not definitely been established.

Referring now to Figure 5, curve N represents the concentration of Ntype of impurity and curve P represents the concentration of P type ofimpurity in the crystal. In this figure the curves are plotted toindicate movement in opposite directions of the two types of impuritiesin an electric field of a given polarity. This is done only to produce aclear picture and to aid in understanding and practicing the inventionand should not be construed as a belief that this is the manner in whichthe impurity movement is influenced. It has been established only that achange in electric field will affect the spatial distribution of theimpurities in the finished crystal.

In Figure 5, at time 0 the concentrations of both types of impuritiesare equal curve N is superimposed on curve P. At time X an electricfield is applied in one direction to produce a rate of change in the conntration of the impurities in the crystal and cause a concentration ofone type of impurity, arbitrarily chosen N, to e:;-- ceed that of theother, resulting in a region of N type material being grown in thecrystal. At time Y the polarity of the electric field is reversed toarrest the rate of change in concentration of the impurities in onedirection and start it in the opposite direction. This is shown inFigure 5 by the fact that the concentrations of the two impuritiesrepresented by curves N and P begin to converge at time Y. The reversedelectric field is continued until the concentration of the secondimpurity exceeds that of the first. This is shown in Figure 5 at thetime following time Z, during which time a region of P typesemi-conductor material is grown on the crystal. The point in Figure 5where the two concentration curves cross is the PN junction and islabelled time alternating of electric field polarity may be used to growseveral P-N junctions in a single crystal.

This method of growing P-N junctions may be contrasted with thegenerally used methods in that in this method the impurities are addedat the beginning of the growing operation and moved by the electricfield to produce the desired predominance of impurities of one type overthe other in the crystal; whereas, in the currently used methods thepredominance of one type of impurity over the other is achieved byadding to the melt greater and greater amounts of the type of impuritynecessary to change the type of material as the crystal is being grown.

Excess carrier lifetime in a semi-conductor is the rate at which thecurrent carriers recombine in the crystal This ' structure and dependsin part on the impurity atoms present. It may be seen that a control ofthe distribution of the impurities in the crystal as it is grown willenable a control to be had over the carriers, and will result inpermitting the growing of a crystal with a known excess carrierlifetime. A crystal of this type is grown in the same manner as acrystal having known specific resistance.

Whlie the method of controlling the distribution of the .impurities in asemi-conductor crystal by the use of an electric field has beendescribed for use as an independent method and has been contrasted withthe currently used methods, this was done only to give a clear pictureand not to imply that the method should be used separately. The methodof the invention may be combined with any of the methods currently inuse as may be expedient.

The reasons for the movement of the impurities by the electric fieldhave not been definitely established, however, there are varioustheoretical reasons that may explain the phenomenon. These theoreticalreasons are included merely as general information to assist inunderstanding and practicing the invention, and the movement of theimpurities may be due to any one or any combination of these reasons ornone of them.

One theory is that the impuriites may exist in the melt to some degreeas ions which are free to move and which may be moved by the forceexerted by the electric field.

Another theory is that the conduction electrons are accelerated by thefield until they strike an impurity atom. When this collision occurs,the recoil imparts motion to the impurity atom.

Still another theory is that the distribution constant for theconcentration of impurities existing in the melt is altered, to somedegree, at the freezing interface and in the first few layers of atomsin the solid, by the electric field.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubsitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in the artwithout departing from the spirit of the invention. It is the intentiontherefore to be limited only as indicated by the scope of the followingclaims.

What is claimed is:

1. A method of growing a semi-conductor crystal comprising the steps ofmaintaining a pure elemental semiconductor material with selectedconductivity type determining impurities in a melted condition,supporting a seed crystal of the same material in engagement with theupper surface of said melt, applying a direct current electric fieldacross said seed crystal and said melt, and lifting said seed crystalslowly to permit a freezing of the melt on the lower end of the crystalwhile said electric field is maintained.

2. A method of producing a semi-conductor crystal of varying specificresistance comprising the steps of immersing an elemental semi-conductorseed crystal in a melt of the same semi-conductor material containingselected conductivity type determining impurities, withdrawing said seedcrystal slowly from said melt so that said material of said melt freezeson said crystal, and passing a direct electric current through thecombination of said melt and said seed crystal during said Withdrawingstep.

3. A method of growing a semi-conductor crystal of predeterminedspecific resistance and given excess carrier lifetime, comprising thesteps of immersing an elemental semi-conductor seed crystal in a melt ofthe same semiconductor material containing selected conductivity typedetermining impurities, withdrawing said seed crystal slowly from saidmelt so that said material of said melt freezes on said crystal, andpassing a direct electric current of selected intensity through thecombination of said melt and said seed crystal during said withdrawingstep.

4. A method of growing a semi-conductor crystal com prising the steps ofapplying heat to a region of a body of elemental semi-conductor materialcontaining selected conductivity type determining impurities, formaintaining said region in a melted condition, progressively coolingsaid melted material from one end to effect a crystalization of thematerial, and applying a direct current electric field across thecombination of said melted material and said crystalized material duringsaid progressive cooling step.

References Cited in the file of this patent UNITED STATES PATENTS2,415,841 Ohl Feb. 18, 1947 2,560,594 Pearson July 17, 1951 2,623,105Shockley et al Dec. 23, 1952 2,651,831 Bond et al Sept. 15, 19532,664,486 Colpitts Dec. 29, 1953 2,683,676 Little et al. July 13, 19542,711,379 Rothstein June 21, 1955

1. A METHOD OF GROWING A SEMI-CONDUCTOR CRYSTAL COMPRISING THE STEPS OFMAINTAINING A PURE ELEMENTAL SEMICONDUCTOR MATERIAL WITH SELECTEDCONDUCTIVITY TYPE DETERMINING IMPURITIES IN A MELTED CONDITION,SUPPORTING A SEED CRYSTAL OF TE SAME MATERIAL IN ENGAGEMENT WITH THEUPPER SURFACE OF SAID MELT, APPLYING A DIRECT CURRENT ELECTRIC FIELDACROSS SAID SEED CRYSTAL AND SAID MELT, AND LIFTING SAID SEED CRYSTALSLOWLY TO PERMIT A FREEZING OF THE MELT ON THE LOWER END OF THE CRYSTALWHILE SAID ELECTRIC FIELD IS MAINTAINED.